1. Field of the Invention
The present invention relates to a method and apparatus for controlling a synchronous motor.
2. Description of the Related Art
A synchronous electric motor (AC motor) is basically structured such that the motor is equipped with a stator with plural-phase armature coils and a rotor with field poles made from permanent magnets or others and currents are supplied to the armature coils to generate torque to rotate the rotor. To control the drive of the synchronous motor in an optimum fashion, it is absolutely required to detect positional information indicating the angular positions (rotational positions) of the rotor (hereinafter, called “angular positions”). The conventional synchronous motor is thus provided with a positional sensor dedicated to detecting the angular positions.
However, loading the positional sensor to a synchronous motor is obliged to be larger in size and higher in production cost. Because use of the positional sensor itself is limited to some environments, applications of the synchronous motor are also confined. With taking this situation into account, by way of example, a structure disclosed in Japanese Patent Laid-open Publication No. 2004-64860 is provided. That is, as shown therein, without employing a positional sensor, the motor is structured to know the angular positions through computation processing of bits of information showing driven states of the motor, which include the values of currents through and voltage to be applied to the armature coils. This type of motor is called “sensor-less synchronous motor.”
In recent years, it has been desired that the magnetic poles of the rotor produce more amounts of flux in order to raise the efficiency of a synchronous motor. This demand is also true of the “sensor-less synchronous motor.
However, in the case of the sensor-less synchronous motor, there is a drawback that a phase current shows a hunting phenomenon of its amplitude, as shown in FIG. 10. The hunting amount X (i.e., an amount of changes in a phase current) is approximately directly proportional to differences ε between the true values of the angular positions and the calculated values (i.e., a phase difference), as shown in FIG. 11. Thus, as the flux from the field poles increases, the gradient of the graph in FIG. 11 increases. Raising the flux for a higher efficiency of the synchronous motor, the hunting amount X will therefore become larger, thereby making the control of the synchronous motor unstable.